Pressure responsive aircraft instrumentation such as altimeters, air speed indicators, and engine pressure ratio or EPR indicators are well known in the art. Examples of such prior art engine pressure ratio indicators are disclosed in U.S. Pat. Nos. 4,003,249; 3,852,741; 3,310,666 and 3,927,307. All of these types of prior art instruments normally employ pressure transducers to measure or detect a desired pressure responsive parameter. For example, in such prior art instruments for determining mach, one pressure transducer is employed to detect the total pressure while another pressure transducer is employed to detect the static pitot pressure, with the measured difference b normally being utilized to obtain q.sub.c via a conventional onboard air data computer. Similarly, in the case of prior art engine pressure ratio indicators, one pressure transducer is employed to detect or measure the numerator component of the engine pressure ratio while another pressure transducer is employed to measure the denominator component of the engine pressure ratio. This information is then normally provided to an onboard air data computer to determine the appropriate EPR indication and display it to the pilot, such as disclosed in U.S. Pat. Nos. 3,886,790; 3,837,220; 4,130,872 and 3,835,701. Because of the required accuracy of the aircraft instrumentation employing pressure transducers, whether they be EPR indicators, altimeters or other instrumentation, costly high precision pressure transducers have been employed for each of the required pressure transducers. Morover, because such instrumentation normally employs several pressure transducers, the cost of such instrumentation has risen considerably. Even with the use of such high precision pressure transducers, errors have still occurred due to such factors as spurious forces or conditions. Thus, the prior art has attempted to provide apparatus that insures that all of the pressure sensitive devices employed in the instrumentation respond identically to spurious forces. An example of such a prior art attempt is disclosed in U.S. Pat. No. 2,866,332 in which a third pressure transducer is employed in addition to the normal transducer pair of an EPR transmitter in order to continuously compensate for errors introduced into the EPR transmitter by the normal pressure transducer pair, such as due to spurious forces or conditions. This is accomplished in the above prior art system by simultaneously applying the combined output associated with all three transducers to the indicator circuitry. The prior art system, however, while compensating for certain errors, still does not avoid the prior art requirement for high precision pressure transducers. These problems are overcome by the present invention in which a pair of low quality pressure transducer is cross-checked by a valving system with a common high precision pressure transducer during alternate cycles of input of the pressure responsive parameters detected or measured by the low precision pressure transducer pair.